Method and apparatus for injection molding



Oct. 4, 1960 G. W. WACKER METHOD AND APPARATUS FOR INJECTION MOLDINGOriginal Filed Jan. 17, 1955 WM.. LAK..

llnited States arent thee 2,954,586 Patented Oct. 4, 1960 METHOD ANDAPPARATUS FOR INJECTION MOLDING 2 Claims. (Cl. 18-30) p This applicationis a division of application 482,321, led January 17, 1955, nowabandoned, and the invention relates to the injection molding of plasticmaterials. More specifically, this invention concerns the method andapparatus for injection molding by the use of packaged molding materialto obtain certain benefits to be derived from the use of preplasticatedmolding material.

The main object of this invention is to provide an injection assemblyfor an injection molding machine which is adapted to use a cylindricalpackage of thermoplastic molding material prepared in means separatefrom the said injection assembly, the said package consisting of adefinite volume of hot, fluid molding material, heated short ofinjection fluidity and enclosed in an integrally formed contaner havingcongealed side and end walls. The said volume is sufficient to exactlylill the mold being worked, the word exactly being advisedly used to`avoid a dis'- sertation on the subject of the elect of heat andpressureon the volume of molding materials.

Heretofore, it has been customary to supply injection molding machineswith hard, granular molding material. This material is forced throughthe restricted passages of the single heating means of the machines bythe injection plunger on the injection stroke. In doing so, up to halfof the thrust of the plunger is dissipated. Only about half of the ratedinjection pressure remains available for actually forcing the heatedmaterial into the mold, up to the instant the mold is filled.

In the practice of this invention, substantially all of the thrust ofthe injection plunger is devoted tothe single task of forcing materialinto the mold. Therefore the injection press using this invention canoperate on injection pressures drastically reduced from orthodoxpractice.

Another advantage is the fact that up to 8() percent of the requiredheat load is already in the package of mate'- rial when it is insertedinto the injection assembly ot' this invention. It follows that theheating requirements of this assembly are substantially below those ofthe orthodox machines, and the heating means of this apparatus canoperate at relatively low` temperatures which greatly reduces the dangerof overheating or degrading material during the operation of the press.

Still another advantage resides in the fact that each package ofmaterial, when inserted in the injection assembly, presents a congealed,distortable end wall to the end of the injection plunger of theassembly. This congealed material serves as an effective seal to preventthe leakage of molding material under pressure past the plunger, therebyavoiding stuck and gauled plungers.v

One more advantage becomes apparent because each package containsexactly the proper volume of material to lill the mold without excessivepacking. For this reason, a bottoming type plungercan be used, andtherclamping tonnage of the press can be substantially reducedf withoutpossibility of flashing the mold.

fIn `the operation of this apparatus, the material flow procedure isfirst in, first out. When the plunger is retracted, no material canremain in that portion of the pressure cylinder which has been swept bythe plunger, nor can material adhere to, or be moved with, the plungerwhen it is retracted. Therefore there is no residue of ,material` leftin the pressure cylinder to give trouble when changing material orcolor.

This improved method of injection molding provides foi two separateperiods ofheating of the plastic material. The lirst heating occursduring Athe preparation of the package as disclosed in the co-pendingdivisional application No. 736,436, filed May 12, 1958l by thisapplicant, entitled Method and Apparatus for Packaging Fluid MoldingMaterial, at which time the material is heated from room temperature toits iiow temperature, or a rise of approximately 250 degrees F. Thislirst heating will not degrade the material because there will be noreason to expose the material for an overlong period to the heatingtemperature, due to the storage method disclosed.

The second heating takes place After the package has been transferred tothe injection molding machine, and after the relatively cool injectionplunger has forced the package thru the relatively cool pressurechamber, during which process the package has lost some heat. To nowraise the temperature of the material constituting the package to itsproper degree of fluidity and uniformity for injection into the moldrequires an additional rise of only approximately 50 degrees, which canbe accomplished in a relatively cool heating chamber. Prolonged exposureto this temperature due to an interruption of the press cycle will notoverheat nor degrade the material.

In the drawings:

Figure 1 is a schematic drawing showing a section through an injectionassembly of this invention, with `the parts in their relative positionsjust after a transfer sleeve holding a package of material has beeninserted into the assembly, with the sleeve alined with the injectionplunger. The location of the air vent is clearly shown. The plunger isnow ready to be advanced to fill the empty mold with twice heatedmaterial.

Figure 2 is an enlarged fragmentary sectional View of the flange end ofthe pressure chamber of the assembly showing the construction of asingle port air vent for purging off trapped air.

Figure 3 is an outline drawing of' a cylindrical package of heatedthermoplastic molding material intended to show its functional featuresinsofar as they relate to the method and apparatus of this invention.

Figure 4 shows a portable transfer sleeve of a type required for theoperation of the apparatus of this invention, and intended to point outfunctional features relating to the method and apparatus of thisinvention.

Description Essentially, the injection assembly Fig. 1, consists of ahousing 53, a pressure cylinder 54, a plunger guide bushing 55, aninjection plunger 56, and a heating chamber assembly 57. An air ventFig. 2, at the discharge end of the pressure cylinder, is also-animportant. part. One or more transfer sleeves Fig. 4 are required tooperate the assembly.

The housing 53 is provided with a throat 63 open at the top, andmachined at the bottom to suit locating lugs 102 on the transfer sleeve50. The housing includes a tunnel bored-at right angles to lthe throatand passing through the throat in a manner to divide the tunnel into along end and a short end.

The pressure cylinder 54 has a tiange at one end, and a bore openat bothends to lit the injection plunger 56. lt fits into the long end of thetunnel. The injection plunge guide bushing 55 has a bore to fit theplunger and is 3 assembled into the short end of the tunnel 71. Both thepressure cylinder and the guide bushing terminate at the throat openingto provide `a space between cylinder and bushing to accommodate thetransfer sleeve. v

The injection plunger 56 is advanced and retracted by a hydraulicallyoperated mechanism (not shown) in the conventional manner. Whenretracted, the plunger is positioned in the guide bushing and clear ofthe transfer sleeve space in the throat. When it is advanced the plungerpasses through the bushing and the throat to operate in the pressurecylinder.

The heating chamber assembly 57 is iitted with a torpedo, a chamber cap,a nozzle, a clamp ring and necessary heating means. It fits over theange of the pressure cylinder, and the clamp ring serves to hold theheating chamber into' leak proof operative association with the angedend of the pressure cylinder.

The air vent Fig. 2 consists of a vent disc 59 having a bore 77 and aflared wall 76 adapted for operative association with the flange of thepressure cylinder, by forming an annular port 61 at the end of the boreof the pressure cylinder when the vent shim l60 is assembled between thellange and disc. The port communicates with an annular groove 72 formedin the flat surface of the ilange, and a passage 62 connects the groovewith the at,- mosphere. The width of the port will not exceed .0 inchand is easily reduced to meet operating conditions by changing the shim.To meet certain conditions of service, the vent can be made with aplurality of discs and shims, to provide more than one port to purge thetrapped air.

It is necessary to keep the vent port 61 relatively cool, so coolingmeans (not shown) are provided to control the operating temperature ofthe pressure cylinder 54 and its flange. 'It is also desirable toprovide means (not shown) to keep the nose of the injection plunger 56cool at all times.

A controlrod 64 is provided in the housing to operate a limit switchadapted to permit the initiation of the advance stroke of the plungerwhen the transfer sleeve is inserted in the throat in proper alinementwith the plunger.

Operation The operation of the injection assembly begins with theselection of a package of heated thermoplastic molding material of asize to t the pressure cylinder of the assembly and of the proper volumeto exactly ill the mold to be operated. This package is prepared in themanner and by the apparatus disclosed in my pending application led May12, 1958, numbered 736,436 and entitled, Method and Apparatus forPackaging Fluid Molding Material. The word exactly is used advisedly toavoid a long dissertation on the effects of temperature and pressure onthe volume of heated plastic molding material.

The selected package is placed in a transfer sleeve tting the assemblyasshown in Fig. 4, and the loaded transfer sleeve is then moved manuallyto the injection press and inserted in the throat of the injectionassembly.

The sleeve has lugs 102 which rest in a machined nest in the bottom ofthe throat to position the sleeve in proper alinement with the retractedinjection plunger of the assembly. By properly locating the sleeve theadvance control rod 64 of the assembly will be depressed to operate alimit switch adapted to permit the initiation of the advance of theinjection plunger.

A preferred method of operatingv a molding unit cornprising a machinefor packaging the heated material and a separate injection moldingmachine adapted to use the packages is to accumulate a bank of packagesin a storage station before starting the injection machine. The packagesare left in the transfer sleeve used to remove them from the packagingmachine during their stay in the storage station while waiting theirturn to be used.

The injection plunger will advance on the injection stroke to passAthrough the transfer sleeve and t9 thereby slide the package out of thesleeve and into the vacant space in the bore of the pressure cylinderleft when the plunger was retracted on theA previous press cycle. Theeffort required to slide the package is negligible and the hot packageis not distorted.

By the time the package encounters the wall of material whichtemporarily blocks the bore, left by the previously worked package, theplunger will have entered the bore. At this stage the plunger will beginto exert thrust against the package first to expand the package tightlyagainst the bore. This will trap a quantity of air which now cannotescape via the plunger clearance. This air will temporarily accumulatein the annular space provided by the chamber 67 of the package, whichspace will subsequently be obliterated by the plunger pressurue.

This air space is adjacent the circular vent port l61 in the bore andthe trapped air will be purged off by passing through the port into thevent groove 72 and passage 62 to the atmosphere. The width of the ventport is adjusted to suit conditions by changing the vent shim 60 whichis made in thicknesses from .010 down to .002 inch, to permit thepassage of air but to block the passage of molding material. The soft,flowable material in thepackage 70 is surrounded by the congealed sidewall 68 and the congealed end walls 65-66, Fig. 3, and cannot reach theport. The congealed material will not enter the port. Under certainservice conditions, more than one port may be required and can beprovided by using a plurality of vent discs, shims and grooves. If thetrapped air is not purged off, the heat developed by the compression ofthe `air can cause black streaks and other defects in the work.

As the plunger continues to advance after purging the air from the bore,the package of material is forced `through the pressure cylinder intothe heating chamber. Inasmuch as the material comprising the package hassubstantially the same compression ratio as the material undergoing thesecond heating in the heating chamber, a

shots, using less mold clamping tonnage than Orthodox machines. Themethod also assures that lthe space in the pressure cylinder which isswept by the plunger is absolutely clean of material to facilitatechanging material or color.

An important feature is the fact that no hot, fluid molding materialcomes in contact with the end -of the injection plunger to leak past theplunger and cause damage by sticking the plunger or gauling both plungerand bore. Instead, the material comprising the end wall of the packagewhich is against the end of the plunger is congealed and distortable toserve as a very effective seal against plunger leakage.

With this method there has been no granular molding material involved tocomplicate volumetric mea-surement of the charge or to roby theinjection plunger of part of its thrust. All the thrust of the plungerVis devoted to the single task of forcing material into the mold.

By this method of using -a preplasticated charge of material, allmaterial which has been heated to uidity is protected against exposureto air which prevents possible discoloration. Y Furthermore, materialwhich has been heated to injection temperature is conned under pressureto avoid undesirable expansion and gassing, up to the instant ofinjection into the mold.

By this method of using a preplasticated charge of material, each shotmay be inspected for color, clarity and -volume before being insertedinto the injection assembly.

And in the event of a prolonged delay in the operation of the injectionpress due to mold or other trouble, the preplasticated material, orpackages, may be salvaged instead of being degraded by too long exposureto high heating temperature.

The plunger of the ordinary injection assembly never quite completes itspossible advance stroke in norm-al operation. Instead the advance of theplunger is halted short of the end of the possible stroke by the backpressure of the material which has been forced into the mold. However,the plunger of this invention does always fully complete its possiblestroke to bring the end of the plunger in line with the circumferentialvent port at the discharge end of the pressure cylinder. This operationof the plunger satisfies two requirements. First, to force `all thematerial which did comprise the package through the pressure cylinderand past the vent port, simultaneously forcing a like volume of materialfrom the heating chamber assembly through the nozzle into the mold.Second, that the congealed end wall of the package abutting the end ofthe plunger be always positioned in the bore of the pressure cylinder toexpose the vent port and assure the proper purging of any Vair trappedduring the expansion of the next worked package.

Fig. 3 of the drawing shows the general structure of the package ofmaterial 70 used in the injection assembly of this invention. Ofcylindrical shape, the outer diameter is made an easy iit into the bore77 of the pressure cylinder, Fig. 2. The full cornered end Wall 65 ispositioned at end 75 of the transfer sleeve 50, Fig. 4, so that it willabut against the end of the plunger 56 during the operation of theinjection assembly. The other end wall 66 is chamfered at 67 to providemomentary access to the vent port v61, Fig. 2, in the bore of thepressure cylinder. An area of relatively soft material re maining afterthe forming of the package is shown at 69. The dotted line 68 denotesthe diffused demarcation between the hot liuid contents of the packageand the cool distortable end walls 65 and 66 and the side w-all.

The Word package as used in this specification shall mean a distortablecylindrical mass of thermoplastic molding material of a diameter to suita pressure cylinder assembly, and containing a volume of materialsufdicient to exactly iill a mold, and comprising material heated to afluid condition short of injection temperature, and enclosed in integralside and end walls of congealed material capable of temporarilymaintaining dimensional stability. The word cool shall mean atemperature at which the molding material is not fluid, or less than 175degrees F.

Fig. 4 of the drawing shows the general structure of a portable transfersleeve used with this invention. It comprises .a tubular section havinga bore 101 into which a package of material fits freely. The tube isprovided with a handle 103 and lugs 102 which locate the sleeve in thethroat 63 of the housing 53 of the injection assembly. A boss 104 isprovided to operate a push rod 64, Fig. 1 to permit the initiation ofthe advance stroke of the plunger when the sleeve is properly located inthe throat. One end is marked for identification so that it may bepositioned toward the plunger when the sleeve is manu-ally inserted inthe throat, to always have the end 65 of the package facing the plunger.The transfer sleeve is used to hold a package and transfer it from thepackage making assembly direct to the injection assembly, or to hold apackage in a storage station prior to moving it to the injectionassembly.

Having described my invention, I claim:

l. In an injection assembly using cylindrical packages of partly heatedthermoplastic molding material, the combination comprising a housinghaving a throat to receive a transfer sleeve and a pressure cylinderassembly tunnel, a pressure cylinder assembly mounted in said tunnel,said assembly having a bore with a circumferential vent port adjacentthe discharge end of said bore, means to cool the pressure cylinderassembly including the vent port, an injection plunger adapted tooperate in the bo-re of the pressure cylinder assembly, means to advancethe plunger its full stroke into the bore to position the end of theplunger in line with the vent port, a heating chamber assembly in owcommunication with the pressure cylinder assembly, means tto maintainthe heating chamber assembly at the temperature necessary to heat itscontents to full injection temperature, an injection nozzle at thedischarge end of the heating chamber assembly, means to maintain theinjection nozzle at injection temperature, and a manually operated,portable transfer sleeve consisting of a cylindrical section, a handleattached to the cylindrical section, locating lugs extending from thecylindrical section, ,a switch rod actuating boss on the cylindricalsection, the said tnansfer sleeve being adapted for insertion into thethroat `of the housing, and for holding a package of material in astorage station.

2. The apparatus of cl-aim 1 in which the pressure cylinder assemblycomprises a substantially tubular cylinder having a straight bore and aflange at the discharge end, a vent groove formed in the end surface offthe flange, a passage connecting the vent groove with the atmosphere, anannular vent shim of selected thickness but not to exceed .010 inch andthe diameter of the cylinder flange with a hole exposing the ventgroove, a vent disc the diameter of the cylinder flange and with a boresame as the cylinder bore, flaring to suit the mouth of lche heatingchamber assembly, the said vent shim and the vent disc when assembledwith the cylinder ange forming a circumferential vent port the width ofthe selected shim in the bore of the pressure cylinder assembly adjacentthe discharge end.

References Cited in the file of this patent UNITED STATES PATENTS2,454,661 Lester et al. Nov. 23, 1948 2,477,258 MacMillin July Z6, 1949FOREIGN PATENTS 629,228 Great Britain Sept. 15, 1959

